PSCC Readings

greengap · Oct 10, 2011

Hi

I have just qualified as an electrician but am wanting to do my wiremans license and have been told to do some inspection and tests before doing the exam so that I can have a practical understanding of the system. With regards to this I did a PSCC test on a simple installation. Using a SUB DB, I connected my green clamp to earth bar, black clamp to neutral bar and red clamp to live incoming. All lock outs were in place and RCD's bypassed to eliminate tripping (I don't have a "no trip" function on my tester). I get a Ze reading of 0.41 ohms and a PSC of 640A. My question is why is the reading so low? Surely this should be in the kA range or does distance play a role i.e. from mini sub to testing location? It is a 3 phase installation so I am told I must double the reading. Is this correct?
Thanks

malcolmsanford · Oct 10, 2011

First of your taking your reading at a sub board so you not getting a Ze reading but a Zdb/Zs reading.

If you take your reading as per ohms law 230/0.41 = 0.56Ka so your 0.64Ka is not that far out.

telectrix · Oct 10, 2011

think you are confusing pscc with pefc. your pefc with a Ze of 0.41ohms should be 230/0.41 = 561A. pscc is measured between L and N ( single phase ) . you double that for 3 phase, or measure ph/ph., in which case the voltage would be 400.

greengap · Oct 11, 2011

Thanks guys for the replies. How much of an affect will it make if I am testing at a Sub DB. This is in a factory so won't be able to do the PSCC test at the main DB...production cannot be stopped. I do understand the double the reading for 3 phase. And yes I am measuring between L - N for the PSCC test. But from what I have read the further you are away from a mini sub/substation the lower the reading will be so I will use the 0.56kA reading.